Said polycarboxylic polymers are known to the prior art. They may be homopolymers of acrylic acid, copolymers of acrylic acid with methacrylic, itaconic, maleic or crotonic acid, copolymers of acrylic acid with methyl, ethyl, propyl or higher esters up to C18-22 of acrylic acid or of methacrylic, itaconic, maleic or crotonic acids, copolymers with other monomers containing a vinyl unsaturation, such as the vinyl esters of linear or branched acids up to C18-20, vinyl esters, and styrene.
Some of these polymers are used in the cosmetic industry under the INCI name of Carbomer.
The homo- and copolymers described above can also be crosslinked with small amounts of ethylenically unsaturated multifunctional monomers such as the allyl ethers of pentaerythritol, trimethylolpropane or sugars.
They are usually prepared by polymerisation in a suitable organic solvent, at atmospheric pressure or a higher pressure in a closed autoclave, using catalysts able to generate free radicals such as peroxides, organic hydroperoxides and azo compounds.
The polymers thus prepared precipitate in the reaction solvent in the form of solid aggregated particles, which are separated from the solvent by filtration and/or direct drying. The powders thus obtained are used as viscosity-controlling agents for aqueous media because they develop very high viscosities after neutralisation.
However, these powders are rather difficult to disperse in water, and sometimes develop an undesirably high viscosity even before neutralisation.
Synthesis processes in the presence of surfactants, generally non-ionic, with hydrophobic-lipophilic balance (HLB) values of between 1 and 12, have been proposed to eliminate these drawbacks. See, for example, U.S. Pat. No. 4,375,533, U.S. Pat. No. 4,419,502 and U.S. Pat. No. 4,420,596.
Interpolymers containing “steric stabilisers”, consisting of surfactants constituted by linear block copolymers with the hydrophobic part not less than 50 Angstroms long, or randomly distributed “comb” copolymers, were disclosed more recently in U.S. Pat. No. 5,288,814. Said steric stabilisers are copolymerised with acrylic monomers, and integrated into the polymer chains after polymerisation.
It has now been discovered that an excellent effect of wettability in water and low viscosity of the aqueous dispersion of the polymers to which this invention relates can be obtained more simply with intimate physical mixtures of polymers or copolymers with small amounts of salts and particular surfactants.